Fast process for the production of fiber preforms

ABSTRACT

A thermomechanical pressing (&#34;TMP&#34;) method for obtaining rapidly a high density (1.2 g/cm 3  or higher) carbon/carbon composite preform from any carbon fiber including PAN-based carbon fibers and industrial coal-tar or petroleum pitches (including synthetic derivatives) having a melting point m.p.=80-350° C. Green preforms may be prepared from mixtures of carbon fibers and pitch, and the resulting mixture is formed to impart a cylindrical shape to the green preform. The green preform is charged into a metallic mold where it is heated, stabilized, pressurized for a period of time, and cooled.

The present invention relates generally to the rapid fabrication of highdensity carbon-carbon composites or preforms used for friction materialsand thermal management systems for automotive and aerospaceapplications.

BACKGROUND OF THE INVENTION

One way to improve the production efficiency of carbon-carbon compositematerials is the development of processes which take advantage of thebenefits of pitch matrix precursors. The main advantages of pitchmatrices reside in their high carbon content (90% and more), relativelyshort process steps, as well as specific material properties resultingfrom their high graphitizability, which provide high thermalconductivity, density and good friction and wear performance.

It is highly desirable to provide rapidly a preform or composite withhigh density (1.2 g/cm³ and above) prior to densification byconventional precursor carbon methods: carbon vapor deposition ordensification ("CVD") and also called carbon vapor infiltration ("CVI"),pitch or resin and their combinations.

SUMMARY OF THE INVENTION

The present invention provides solutions to the above by providing amethod of producing rapidly a carbon-carbon composite made from a greenpreform comprising carbon fibers and at least one pitch, comprising thesteps of:

(a) heating the green preform to a temperature of approximately 450° C.,then increasing the temperature at a lesser rate up to approximately520° C.;

(b) holding at a temperature within the range of approximately 450-520°C. for a period of time;

(c) pressing the preform at approximately 520° C. or higher;

(d) heating the preform to within the range of approximately 520°C.-1,000° C. followed by a soak; and

(e) cooling to provide the composite.

We have disclosed a thermomechanical pressing ("TMP") method forobtaining rapidly a carbon/carbon composite material using any carbonfiber including polyacrylonitrile ("PAN") based carbon fibers andindustrial coal-tar or petroleum pitches (including syntheticderivatives) having a melting point m.p.=80-350° C. Generally, a greenpreform may be prepared from mixtures of chopped carbon fibers andpitch, for example a mixture prepared from 1 part by weight of carbonfibers chopped to the length of Lf=10-50 mm and 1-3 parts by weight ofpitch. The resulting mixture is pressed to impart a cylindrical shape tothe green preform. The green preform is charged into a metallic moldwhere it is heated, pressurized, stabilized, and cooled.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail below with reference to thedrawings in which:

FIG. 1 is a graph of the thermal decomposition of coal tar petroleumpitches and mixtures;

FIG. 2 is a graph of the thermal decomposition of pitches and lowtemperature carbon fiber mixtures;

FIG. 3 is a graph of the thermal decomposition of pitches and hightemperature carbon fiber mixtures; and

FIG. 4 is a graph of the temperature/pressure/time profile used duringthe TMP process.

DETAILED DESCRIPTION OF THE INVENTION

Among processes for improving the production efficiency of pitch-basedcarbon-carbon composite materials, we think it expedient, with regard tothe requirements for friction materials, to use thermal stabilizationmethods in combination with a forced mechanical contraction or pressingof the material during pyrolysis. The method developed is referred to asthermomechanical pressing ("TMP").

A pitch matrix and PAN fiber reinforcement were chosen for the TMPprocess. At present, commercially produced coal tar pitches havesoftening points Ts=65-75° C. and Ts=135-140° C. and toluene insolubles(α-fraction) content of 20-25 wt. % and 45-54 wt. %, respectively, formedium and high-temperature pitches.

Medium pitches enable the preparation of high-quality "green" bodieswith a low coke yield. High-temperature pitches have a higher cokeresidue of about 55-60%, but their use presents difficulties because attemperatures of 120-280° C. they do not wet carbon substrates thuspreventing the use of conventional methods to press "green" bodies.

Therefore, it is of interest to determine if an industrial petroleumpitch with Ts=140° C. can be used in TMP processes. Such pitches havecoke residue somewhat lower than a high-temperature coal tar pitch, butconsiderably higher than a medium pitch and experimental coal tar pitchwith Ts-101° C. and α-fraction content of 33.6 wt. %. It was found thatthe industrial petroleum pitch exhibited better wetting ability comparedto high-temperature coal-tar pitches.

In addition, high-temperature petroleum and medium coal-tar pitches weremixed by comelting and the resulting mixtures were studied in an effortto improve wetting characteristics without essential loss in the cokeresidue yield.

Based on laboratory results, mixtures of high-temperature petroleum andmedium coal-tar pitches were prepared by comilling, and the softeningtemperature of the mixtures of pitches was approximately 100-110° C.

The following pitches and their mixtures were investigated:

1. Pitch grade "A" GOST 10200-83, a medium coal tar pitch, Ts=74° C.

2. An experimental coal tar pitch with elevated softening temperature,Ts=101° C.

3. Pitch grade "I" specs. 14-6-84-72, a high-temperature coal tar pitch,Ts=140° C.

4. Pitch grade "IIHII CB" specs. 48-4807-287-94, a high-temperaturepetroleum pitch, Ts=140° C.

5. mixture of pitches 1 and 4 in the ratio 0.5:0.5 a laboratory sample

6. mixture of pitches 1 and 4 in the ratio 0.3:0.7 a laboratory sample

7. mixture of pitches 1 and 4 in the ratio 0.45:0.55 an industrialsample

8. mixture of pitches 1 and 3 in the ratio 0.5:0.5 a laboratory sample

The composition and properties of pitches were characterized in terms ofthe softening point, coke residue yield, and toluene insoluble fractioncontent. In addition, the molecular weight distribution, thermaldecomposition behavior in the temperature range from 200 to 800° C. andthe onset wetting temperature of carbon substrate (at a 90 deg. contactangle) were also determined. Pitch characteristics are presented inTable A.

                                      TABLE A                                     __________________________________________________________________________    Composition and properties of the pitches.                                              Numbers of pitches                                                  Property  1   2   3   4   5   6   7   8                                       __________________________________________________________________________    Softening point ° C.                                                             74  101 140 140 120 118 110 99                                      (Ring and rod)                                                                Coke residual, %                                                                        36.3                                                                              48.0                                                                              60.0                                                                              52.6                                                                              44.5                                                                              47.6                                                                              48.0                                                                              50.0                                    Fraction of toluene                                                                     22.0                                                                              33.6                                                                              54.0                                                                              29.3                                                                              25.6                                                                              27.1                                                                              25.8                                                                              38.0                                    insoluble substances                                                          in the, %                                                                     The onset wetting                                                                       105 140 --* 260 205 230 --*                                         temperature, ° C.                                                      Thermal analysis:                                                             Mass loss, (%)                                                                in the range                                                                  up to 360° C.                                                                    21.0                                                                              9.0 6.5 8.4 14.8                                                                              13.0                                                                              11.5                                                                              12.9                                    360-480° C.                                                                      32.6                                                                              24.8                                                                              23.7                                                                              25.4                                                                              27.6                                                                              28.2                                                                              28.8                                                                              28.8                                    480-620° C.                                                                      5.6 9.7 6.5 8.7 9.3 8.8 9.3 7.5                                     Coke residual                                                                           39.4                                                                              52.1                                                                              63.3                                                                              56.0                                                                              46.9                                                                              46.8                                                                              49.6                                                                              50.2                                    at 800° C., %                                                          Mesophase formation                                                                     520 500 510 505 515 510 495 515                                     temperature ° C.                                                       Molecular weight                                                              distribution:                                                                 a) Mw, a.e.m.                                                                           327 554 421 551                                                     δ) Mn, a.e.m.                                                                     234 329 297 359                                                     Content of substances                                                                   80.0                                                                              26.1                                                                              28.4                                                                              21.8                                                    with molecular weight                                                         under 300 a.m.u., %                                                           Mw/Mn     1.40                                                                              1.58                                                                              1.39                                                                              1.53                                                    __________________________________________________________________________     *No wetting at 120-280° C. is observed.                           

It is seen from Table A that the petroleum pitch is similar to the coattar pitch in the α-fraction content, basic temperature ranges of massloss, molecular weight distribution, and mesophase formationtemperature, superior in the coke residue yield and somewhat inferior inthe wetting characteristics. The addition of the medium pitch in theamount of 30-50 wt. % to the petroleum pitch improves wettingcharacteristics but does result in the loss in coke residue. Theproperties of industrial mixtures of pitches 1 and 4 (coke residueyield, α-fraction content, mesophase transition temperature, thermaldecomposition behavior) are similar to laboratory sample mixture 6 andexperimental coal tar pitch 2, but are inferior to the latter in wettingcharacteristics because mechanical stirring on comilling failed toprovide the averaging effect in a mixture composition.

At the same time, molecular weight distribution in the petroleum pitchis quite close to that in the experimental coal tar pitch 2, both in theaverage molecular weight values (Mw and Mn) and low molecular compoundsfraction of the total mass. Thermal decomposition behaviors (FIG. 1) andtemperature ranges of mesophase transition in these pitches are alsoclose between themselves which allows us to suggest the use of thepetroleum pitch per se without medium pitch additions in the TMPprocess.

In an effort to study the effect of pitch nature on forming thestructure and properties of a "primary" matrix in the TMP process,interactions between samples of petroleum and coal tar pitches and theirmixtures with carbon fibers heat treated at 1000° C. and VPR-19C carbonfibers heat treated at 2800° C., were investigated. Pitches 2 and 4 andpitch mixtures 5-8 were used for the investigation. Fiber weightfractions in the compositions were 60%. Characteristics of thecompositions are presented in Table B.

The caking index, defined as the coke residue gain in a composition (ΔK%) relative to the coke residue yield from the unreinforced pitch, wasmeasured as an indication of the interaction between the fiber andpitch. Mass loss changes in the temperature range from 20 to 800° C.were also determined and compared to those in pure pitches.

As evident from Table B, the highest coke residue gain (see ΔK %) in thepresence of carbonized fibers was obtained in the composition containingexperimental coal tar pitch 2, whereas the highest coke residue yield(see coke residue %) was reached in the composition based on petroleumpitch 4. Among pitch mixtures, the best results in terms of coke residueyield were obtained in the composition prepared with pitch mixture 6. Inthe composition containing VPR-19C graphitized carbon fibers and pitch4, due to the increased mass loss in the temperature ranges up to 360°C. and from 480 to 620° C., the coke residue yield was higher than inthe pure pitch but lower than in the composition with carbonized fibers.

It should be noted that the use of coal tar pitch mixture 8 as a matrixin compositions prepared with carbonized fibers provides a lowering ofthe softening point, a decrease in the α-fraction content, and improvedwettability of the pitch but gives no gain in the coke residue comparedto the petroleum pitch and its mixtures (Table B, FIG. 2).

In compositions reinforced with the VPR-19C graphitized PAN fiber, thecoal tar pitch mixture 8 exhibited higher coke yield and caking indexcompared to pitch 4 and pitch mixture 7 (see FIG. 3).

These results indicate that the experimental coal tar pitch withincreased softening point (pitch 2), petroleum pitch with Ts=140° C. andcoke residue yield of at least 56 wt % (pitch 4) and mixtures of thelatter with medium coal tar pitch in the ratio of about 60:40 (pitchmixture 7), exhibit attractive properties when mixed with carbonized andgraphitized carbon fibers under TMP process conditions. Pitch mixture 8can be used in compositions with graphitized fibers.

                  TABLE B                                                         ______________________________________                                        Properties of pitch-fiber compositions                                                     Compositions with pitch samples                                  Properties     2      4      5    6    7    8                                 ______________________________________                                        Carbonized fibers of                                                                         17.2   8.0    15.7 12.0 10.8 11.3                              VPR-19C type, ΔK %                                                      Thermal analysis:                                                             Mass loss, % in the                                                           temperature range, ° C.                                                up to 360      15.9   9.3    21.6 17.3 17.5 21.3                              360-480        15.4   19.6   17.7 17.8 18.1 17.4                              480-620        6.2    6.2    4.0  4.4  8.2  7.4                               Coke residue at 800° C., %                                                            61.1   62.8   55.1 58.5 54.2 51.4                              Graphitized fibers of 7.2              11.6 14.7                              VPR-19C type, ΔK %                                                      Thermal analysis:                                                             Mass loss, % in the                                                           temperature range, ° C.                                                up to 360             15.4             18.1 20.4                              360-480               17.1             16.6 13.1                              480-620               9.3              8.5  5.3                               Coke residue at 800° C., %                                                                   57.3             56.3 59.6                              ______________________________________                                    

The optimum composition of the green preform used in the TMP processrequires evaluation of the friction and wear performance of the finalproduct as well as technical and economic considerations.

Previous investigations have indicated that that the highest frictioncharacteristics are offered by high modulus graphitized fibers. However,the use of carbonized fibers is advantageous due to their low cost.Therefore, the following fibers were used as a reinforcing filler forthe model friction carbon-carbon composite material:

Type 1. PAN fiber VPR-19C, heat treatment temperature To≅2800° C., theaverage density, α=1.92 g/cm³, fiber length L<0.5 mm

Type 2. PAN fiber VMN-4, heat treatment temperature To≅2000° C., theaverage density α=1.70 g/cm³, fiber length L=30-40 mm.

Type 3. Carbonized PAN fibers, heat treatment temperature To≅1000° C.,the average density α=1.77 g/cm³, fiber length L=20-30 mm.

All the fibers were derived from PAN fibers. "Green bodies" of theTermar type materials were used for the development of TMP. "Termar" isa tradename for carbon-carbon composite friction material developed byNIIgrafit and produced at the Electrode Plant, both in Moscow, Russia.

Six versions of green bodies were produced from the selected types offibers and pitches and their mixtures using conventional fabricationmethods. The weight ratio of binder (pitch): filler (fiber) for thepremix was 0.40:0.60 in case of short cut fibers (Type 1), and 0.5:0.5in case of long fibers, respectively. To determine the actual carbonfiber:pitch matrix ratio in the green body appeared to be difficult.Sample discs of 126 mm diameter and 25-40 mm thick were cut fromfull-scale discs of the Termar type material of the dimensions: outerdiameter 490 mm, inner diameter 230 mm, thickness 25-40 mm. Eightsamples were made from one full-scale disc. The apparent density andvolume of the samples were determined via hydrostatic weighing.

The samples were then subjected to a thermomechanical pressing ("TMP"),as described below, in a special metal mandrel fitted with an externalelectric heating. The mandrel capacity for one charge was 5-6 samples.

The green preform samples were heated at an arbitrary rate (such as 3°C./min.) to approximately 470° C., after which the temperature wasincreased at a heating rate of 1° C./minute. In order to thermallystabilize the pitch, the preform was held at a stabilization temperaturewithin the range of 450-520° C. for a soak time, for example, ofone-half to one hour, to reduce the volatile content and increase theviscosity of the matrix pitch during the application of pressure. Thevolatiles content of the pitch can determine the soak temperature andtime. Thermal stabilization is accomplished at a low pressure (forexample, about 1-3 MPa). Upon attaining approximately 520° C., thesamples were subjected to a mechanical pressurization within the rangeof 25-40 MPa (this increased pressurization can be done, for example, in10-20 minutes). The samples continued to be heated to a temperature of600±20° C. and maintained at that temperature for a period of time, forexample a 1.5-2 hours soak, the specific pressure being 25-37.5 Mpa, inorder to remove volatiles and convert the pitch to a solid carbon. Thetemperature at the soak is envisioned to be within the range of520-1000° C. Pressure was maintained during the cooling of the sampleswhich was through natural cooling. See FIG. 4 which illustrates thetypical temperature, pressure, and time profile for the TMP process.

The samples were removed from the mandrel, and their mass, volume, andapparent density were determined. The mass loss during TMP wascalculated as follows:

    ΔP=(Pi-Pf)*100%/Pi,                                  (1)

where

Pi is the initial sample mass,

Pf is the final sample mass after TMP,

and the volume shrinkage after TMP was calculated as follows:

    ΔV=(Vi-Vf)*100%/Vi,                                  (2)

where

Vi is the initial sample volume,

Vf is the final sample volume after TMP. The results are presented belowin Table C

                                      TABLE C                                     __________________________________________________________________________    Characterization of various types of composites after TMP.                                  Average                                                                              Average                                                                density of                                                                           density of  Volume                                       Composition of a material                                                                   initial sample                                                                       sample after                                                                         Mass loss                                                                          shrinkage                                    Technological features of TMP                                                               dk, g/cm.sup.3                                                                       TMP dk, g/cm.sup.3                                                                   Δ P, %                                                                       Δ V, %                                 __________________________________________________________________________    Version 1     1.334 ± 0.075                                                                      1.44 ± 0.026                                                                     15.8 ± 3.8                                                                      22.2 ± 4.3                                Fiber Type 1 + pitch mixture 8                                                P = 25 MPa                                                                    without thermal stabilization                                                 Version 2     1.400 ± 0.051                                                                     1.457 ± 0.020                                                                     21.3 ± 2.9                                                                      24.4 ± 2.5                                Fiber Type 1 + pitch 2                                                        P = 25 MPa                                                                    without thermal stabilization                                                 Version 3     1.434 ± 0.028                                                                      1.55 ± 0.025                                                                     21.1 ± 3.7                                                                      27.0 ± 3.9                                Fiber Type 1 + pitch 2                                                        P = 25 MPa                                                                    without thermal stabilization                                                 Version 4     1.437 ± 0.01                                                                      1.638 ± 0.029                                                                     17.8 ± 2.4                                                                      27.9 ± 1.7                                Fiber Type 1 + pitch 2                                                        P = 37.5 MPa                                                                  with thermal stabilization                                                    Version 5     1.391 ± 0.031                                                                     1.470 ± 0.019                                                                     18.8 ± 7.5                                                                      23.2 ± 5.5                                Fiber type 2 + pitch 2                                                        P = 25 MPa                                                                    with thermal stabilization                                                    Version 6     1.256 ± 0.09                                                                      1.363 ± 0.033                                                                     19.6 ± 6.9                                                                      26.1 ± 6.4                                Fiber Type 3 + mixture 7                                                      P = 25 MPa                                                                    with thermal stabilization                                                    __________________________________________________________________________

The main purpose of this work was to develop and determine theefficiency of the TMP method, and to consider the affect of variousprocess and material parameters on the TMP process. The experimentsconducted have shown that TMP is realizable, i.e., it is practicable toobtain, at an accelerated rate, carbon-carbon composite materials of1.5-1.65 g/cm³ density using no modifying additives and with relativelyinexpensive equipment. The present invention comprises composites orpreforms with a high density of 1.2 gm/cm³ or greater which can beobtained according to the desired end application. Such preforms cancomprise a carbon-carbon composite, with or without graphitization heattreatment, as an end product for desired applications.

Comparison of the versions of combining carbon fibers with matrixpitches, from the viewpoint of obtaining green bodies, has shown thatthe mixture for versions 2, 3, 4 (fiber type 1+pitch 2) is the mostuseful composition. The density variation for the green body is within2-4%.

Furthermore, it should be pointed out that properties of the startingpitch material and the filler used, as noted above, markedly affect thecarbon-carbon composite material quality and density after TMP. The bestresults were obtained using a fine-dispersion filler fiber (Type 1)which has not only a higher density and a larger surface due togrinding, but also a higher specific surface area conditioned by a highheat treatment temperature. These parameters appear to be important forthe process of combined (fiber/matrix) caking under TMP.

Comparison of the experimental results have shown that TMP with thermalstabilization of the pitch (versions 3-5) is more efficient than theprocesses without thermal stabilization predominantly because of theprevention of bloating. As follows from considering the reasons of thisphenomenon, the coke yield after TMP with the thermal stabilizationexposure is not substantially different as compared to the controlprocesses without thermal stabilization. This is evident from the massloss index Δ P which indirectly reflects the carbon solid residue. Thecloseness of the Δ P values for all versions of the investigatedcompositions and conditions of processing is explained by the fact thatthe conditions of TMP do not inhibit a free evolution of volatiles underpolycondensation and pyrolysis of each pitch type. Thus, the coke yieldduring TMP depends predominantly on properties of the initial pitch andfibers used.

It is our opinion that the efficiency of the thermal stabilizationexposure or soak resides in increasing the viscosity throughout thesample. The viscosity increase during thermochemical transformationsmakes it possible to decrease the amount of pitch being squeezed out ofa sample under pressure, to decrease the composite porosity level, andthus prevent bloating.

Accordingly, the purpose of the TMP process optimization was to find atemperature-time region during or after the pitch thermal stabilizationwherein the pressure application would not cause binder to be squeezedout of a preform and would be applied to a material which did not loseits ability to cake. The test results show that the purpose has beenbest attained for the compositions based on pitch 2 (Versions 3-5).

As regards the pressure effect on the material quality, it was foundthat the optimal results were obtained when using pressure of about 25Mpa. The pressure increase in TMP led to the density increase, as seenfrom Table B (Version 4), however at the same time the number of defectsin the final material grew in the form of cracks, delaminations andvoids which can be explained by the extreme increase of the fibercontent and stresses arising at considerable strains of the high-modulusreinforcing fibrous filler.

The use of TMP for the composition: fiber type 3+ mixture 7 (Version 6)should be considered separately. The behavior of green preformscomprised of petroleum and coal tar pitch mixtures and carbonized fibersemployed as the filler made it impossible to obtain high-quality greensemiproducts by conventional methods. The green bodies' initial lowdensity (on some samples 0.9-1.0 g/cm³) owing to an insufficient pitchimpregnation brought about additional difficulties in the TMP. BecauseVersion 6 includes pitches of different origins, difficulties resultedfrom the incompatability of the pitches' pyrolysis and thermalstabilization characteristics. This prevented the obtaining of resultsreflecting the potentialities of Version 6. For Version 6, newapproaches are required in order to obtain green bodies and for theoptimization of the thermal stabilization conditions during TMP.

We claim:
 1. A method of producing rapidly a high density, partiallyporous preform for subsequent densification and made from a greenpreform comprising carbon fibers and at least one pitch, comprising thesteps of:(a) heating the green preform to a temperature in the range ofapproximately 450-520° C.; (b) holding at a temperature within the rangeof 450-520° C. in order to effect a desired mesophase content anddensity including porosity; (c) pressing the preform at 520° C. orhigher; (d) heating the preform to within the range of 520° C.-1,000° C.followed by a soak; and (e) cooling the preform.
 2. The method of claim1, wherein the heating in step (d) is to 600±20° C.
 3. The method ofclaim 1, wherein the fibers comprise PAN fibers.
 4. The method of claim1, wherein the pitch comprises a coal tar pitch with elevated softeningtemperature above 90° C.
 5. The method of claim 1, wherein the highdensity preform has a density in the range of 1.2-1.65 g/cm³.
 6. Themethod of claim 1, wherein the temperature is held for up to one hour instep (b).
 7. The method of claim 1, wherein the temperature is increasedat a rate of 1° C./minute up to the range in which the temperature isheld in step (b).
 8. The method of claim 1, wherein the preform ispressed in step (c) to a pressure within the range of 25-40 MPa.
 9. Themethod of claim 8, wherein the pressure is maintained within the rangeof 25-40 MPa during cooling.
 10. The method of claim 1, wherein thegreen preform is made from combining the carbon fibers and pitch priorto the heating of step (a).
 11. The method of claim 1, wherein the pitchcomprises less than 100% converted mesophase pitch.
 12. A method ofproducing rapidly a high density, partially porous preform forsubsequent densification and made from a green preform comprising carbonfibers and at least one pitch, comprising the steps of:(a) heating thegreen preform to a temperature in the range of approximately 450 to 520°C.; (b) holding at a temperature within the range of 450-520° C. inorder to effect a desired mesophase content and density; (c) pressingthe preform at 520° C. or higher; (d) heating the preform to within therange of 520° C.-1,000° C. followed by a soak of approximately 1.5 to 2hours; and (e) cooling the preform.